1. Field of the Invention
The present invention relates to surface particle detection, and more specifically, it relates to the detection of defects on lithographic mask blanks.
2. Description of Related Art
The proposed 1999 SIA Technology Roadmap for Semiconductors is accelerating the reduction in dense line critical dimensions (CDs) to 23 nm by the year 2011. This will put a tremendous burden on mask fabrication, particularly in the area of defect detection and reduction. Mask defects as small as one-eighth the equivalent CD are printable and may cause chip failure. Table 1 shows the maximum permissible defect size for each lithography generation out to the year 2011.
TABLE 1 Year of first 1999 2002 2005 2008 2011 2014 shipment Generation (nm) 180 130 100 70 50 35 Maximum mask 90 65 50 35 25 18 defect size (nm) (assuming a lithography tool magnification of 0.25)
A new infrastructure for mask inspection will be required to keep pace with this aggressive roadmap. Depending on the specific lithography used for a particular generation, mask inspection specifics may change, but the methodology will essentially remain the same. Mask blanks will have to undergo 100% area inspection for defects larger than a maximum acceptable size. Since masks are becoming a significant cost factor in the cost of ownership of lithography tools, this is a critical step--patterning defective mask blanks would be an economic disaster.
Inspecting mask blanks can be approached differently than patterned masks. Inspection does not necessarily have to be done at-wavelength since defects at the mask blank level will interact with visible light. Techniques using visible light are appealing because they are familiar to the user, relatively straightforward to manufacture and, if designed properly, extendable over many generations.
Current wafer inspection tools could play this role if silicon wafers are used as the mask blanks, but this is unlikely due to unfavorable thermal properties. Even then, detection of defects smaller than 100 nm has not been demonstrated with these tools. Wafer inspection tools operate by measuring the intensity of the light scattered by a surface defect. FIG. 1 shows a typical optical system used for commercial wafer inspection. However, scatter decreases as the sixth power of the defect size, so as the critical defect size decreases, defects become extremely difficult to detect, i.e., as the defect size decreases, the scatter decreases by the fractional decrease in defect size to the 6.sup.th power. Additionally, this scattered intensity inspection technique cannot distinguish surface defects from internal defects in transparent substrates such as ULE, a prime candidate for future mask blanks. As shown in FIG. 1, incident light beam 10 having S and P polarizations, is directed onto a mask blank 12 at the site of a defect 14. Scattered light 16, scattered from defect 14 is collected with collector mirrors 18 and 19 and directed to detector 20. This technique cannot distinguish whether the defect is located on the surface or within the transparent substrate.